Significance Epidemiologically, breast-fed infants contract fewer infections and have infections of shorter duration. These differences may be due to differences in composition between breast milk and formula. Glycomacropeptide, (GMP, a highly negatively charged carbohydrate found in milk) may mimic the glycans that bacteria use for attachment in the small intestine and thus may inhibit attachment and colonization, thereby preventing infections. Alpha-lactalbumin, (ALA, a predominant milk protein) is believed to make a major contribution to the balanced plasma amino acid pattern of breast-fed infants and may aid in the absorption of calcium and zinc. The effects of GMP or ALA supplementation will be assessed by the capacity to respond to a microbial challenge as well as by tracing mineral and amino status. Objectives To study the effects of glycomacropeptide (GMP) and alpha-lactalbumin (ALA) on the colonization of pathogenic bacteria as well as anthropometry, hematology and indices of mineral nutriture in infant monkeys. Results Although no differences in length were observed, a significant differences in weight between animals fed GMP and breast-fed infants was found. GMP and ALA resulted in significant increases in serum zinc as 2, 3, 4 and 5 months of age. The absorption of zinc was higher in the animals receiving ALA or GMP as compared with breast-fed infants. Although there were no differences in thymus size, animals receiving ALA or breast milk responded differently to inoculation of E. coli as assessed by changes in differential white blood cell count. Future Directions To evaluate the effect of other milk protein fractions on various pathogens. KEY WORDS glycomacropeptide, alpha-lactalbumin, E. coli, pathogens, prebiotic, infant nutrition FUNDING Private Sector